In vitro studies carried out with secretion granules isolated from rabbit parotid glands have shown that low concentrations of Ca 2 ion cause granule lysis and two preceding events; activation of phospholipase A2 activity and a rearrangement of granule membrane components observable in freeze fractured preparations. Phospholipase A2 has been purified from isoproterenol-stimulated rabbit parotid secretion. The research proposed for the coming year includes: a) enzymatic characterization of the purified phospholipase, especially its activity against complex lipid membranes using quantitative densitometry of reaction products resolved by thin layer chromatography; b) improvement of the intactness of isolated secretion granules thereby facilitating further investigation of their interaction with Ca 2 ion, ATP, and other cellular organelles; c) isolation of a cellular fraction enriched in parotid apical plasmamembrane for use as a fusion partner for secretion granules in in vitro studies; d) quantitative characterization of Ca-induced rearrangement of granule membrane components observable by freeze fracture and search for an in vivo correlate; e) characterization of a phospholipid acylation process enhanced in mast cells by stimulants of histamine discharge and extension to other exocrine secretory systems. It is hoped that these studies will provide insight into the vivo mechanism of exocytosis.